1. Field of the Invention
The present invention relates to a solid-state imaging device having a diffusion preventing film for preventing diffusion of metal constituting wires formed on or above a semiconductor substrate in which a plurality of photoelectric transducers are formed. The present invention also relates to a method for making the solid-state imaging device.
2. Description of the Related Art
In recent years, as the semiconductor integrated circuits become finer, copper (Cu) wires are increasingly used, which activates the development of Cu-wiring processes. Formation of Cu wires is carried out by a damascene process with high production efficiency.
According to the damascene process, wiring interlayer films are formed by a chemical vapor deposition (CVD) process or an application process and dry-etched to form openings for trenches and vias. Subsequently, a metal, such as Cu, W, Al, Ag, Ta, TaN, Ti, or TiN, is buried in the openings by, for example, sputtering, CVD, or plating, and the excess metal is removed by chemical mechanical polishing (CMP). A diffusion preventing film composed of silicon nitride, silicon carbide, or the like is then formed on the polished surface to provide capping. This process is repeated when a multilevel Cu wiring structure is desired.
The diffusion preventing film is a film deposited after CMP to prevent diffusion of the metal of the metal wires. The wiring interlayer film is a film for forming through holes that connect metal wires of different levels to each other, gate electrodes to the metal wires of different levels, metal wires to the semiconductor substrate, etc.
The wiring interlayer film and the diffusion preventing film are usually composed of different materials since their target properties are different. For example, a material having a low dielectric constant is used in the wiring interlayer film to decrease the capacitance with respect to an adjacent wire. In contrast, the diffusion preventing film is desired to prevent metals of wires from migrating into transistor sections in the semiconductor substrate through the wiring interlayer film and to thereby prevent degradation of the properties such as leak current, white spot, black spot, etc.
As is discussed above, the wiring interlayer film and the diffusion preventing film are usually composed of different materials and exhibit different optical properties. For example, the wiring interlayer film is composed of silicon oxide and has a refractive index of about 1.6. In contrast, the diffusion preventing film is composed of silicon nitride and has a refractive index of about 2.0.
Light is reflected either partially or totally depending on the angle of incidence at the interface between films having different refractive indices. In a solid-state imaging device having Cu wires formed by a damascene process, incident light is reflected due to the difference in refractive index between the wiring interlayer film and the diffusion preventing film. This disadvantageously results in a decreased amount of light reaching the sensor, i.e., a photoelectric transducer such as a photodiode.
Japanese Unexamined Patent Application Publication No. 2003-324189 discloses a solid-state imaging device in which reflection of light due to the difference in refractive index between the wiring interlayer film and the diffusion preventing film is avoided by selectively removing a Cu-diffusion preventing film in the region overlaying the sensor in the semiconductor substrate as viewed in the thickness direction or the substrate, and then burying a film in that region.